The present invention relates to an improvement in the contact sulfuric acid process. More particularly, the present invention relates to an improvement in the contact sulfuric acid process employing the double conversion-double absorption steps operating on cold sulfur dioxide-containing feed gas obtained from a wet gas purification plant.
In the contact sulfuric acid process, a gas stream containing both sulfur dioxide and free oxygen is passed through a catalyst, after having been preheated to the kindling temperature of the catalyst, if necessary, to effect oxidation of the sulfur dioxide to form sulfur trioxide, followed by cooling the gas stream to temperature within the range of about 400.degree. to about 550.degree. F and contacting it with strong sulfuric acid to effect selective absorption of the sulfur trioxide. The oxidation of sulfur dioxide to sulfur trioxide is an equilibrium reaction; complete oxidation of the sulfur dioxide cannot be achieved in the presence of the sulfur trioxide which is being formed. However, the equilibrium can be shifted toward the sulfur trioxide by removing the sulfur trioxide from the reaction mixture. Therefore, it is common practice to subject the gas stream from which sulfur trioxide has been selectively removed by scrubbing with strong sulfuric acid to a second conversion by passing it through a catalyst mass, after preheating it to the kindling temperature of the catalyst. Thereafter it is again cooled and passed through a second absorber wherein it is contacted with strong sulfuric acid to scrub sulfur trioxide therefrom. The exit gas from the second absorber, which now contains only trace amounts of unconverted sulfur dioxide, is vented to the atmosphere.
As previously noted, the feed gas stream containing both sulfur dioxide and free oxygen must, before introduction into the catalyst mass, be heated to at least the kindling temperature of the catalyst which, in the case of the commonly employed vanadium catalyst, is in the order of about 780.degree. to about 800.degree. F. In a sulfuric acid plant wherein the sulfur dioxide-containing feed gas stream is obtained by combustion of brimstone, this presents no problem since the gases obtained by combustion of brimstone are at a temperature above that kindling temperature, and excess heat can be recovered from the gas stream, as by passing it through a waste heat boiler prior to introducing it into the converter containing the catalyst. Also, the oxidation of sulfur dioxide to sulfur trioxide is an exothermic reaction, so that heat is generated in the conversion, which can be recovered, if desired.
In a sulfuric acid plant employing the double conversion-double absorption process steps, as above described, the gases obtained from the intermediate absorber must be preheated prior to introducing them into the second conversion stage. This can be effected under utilization of the heat generated in the second conversion stage, as shown by Furkert et al. in U.S. Pat. No. 3,443,896. However, since the temperature differentials involved here are relatively small, impracticably large heat exchangers of the shell-and-tube type would be required, and even then the heat available from reaction in the secondary stage is generally insufficient to make the process self-sustaining. For that reason, Furkert et al. suggested addition of pure, dry sulfur in the form of solid granules or in liquid form to the gas stream prior to its introduction into the secondary conversion stage, so that the sulfur melts and is ignited immediately and very quickly delivers the required heat in order to maintain the oxidation in the second conversion stage. Furkert et al.'s suggestion, while operable, presents certain equipment problems in that it requires special means for introducing the elemental sulfur. Furthermore, elemental sulfur may not be available at the plant, for example, when the plant is operating on sulfur dioxide-containing gas obtained by roasting of sulfur ores.
Disadvantages of use of shell-and-tube type heat exchangers for preheating the feed gas stream to the secondary conversion stage are overcome by employing heat sink type heat exchangers, as described by Mareski et al. Canadian Pat. No. 975,927. The higher heat recovery possible with less expensive and less corrosion susceptible pebble bed type heat exchangers as described by Mareski et al. may indeed avoid need for burning sulfur in the gas stream, as required by Furkert et al., in the event the gas stream being fed to the secondary conversion stage contains at least about one percent of sulfur dioxide. Nevertheless, the pebble bed heat exchangers required by Mareski et al. are bulky, and at least two of these are required. These are alternately heated and cooled by alternating the direction of the gases flowing through them, so that they require relatively close operator attention.
It has also been proposed to employ shell-and-tube type heat exchangers for preheating the feed gas stream to the secondary conversion stage to effect indirect heat exchange contact with exit gases from that stage, as described by Furkert et al., and to supply the additional amount of heat which may be required to bring the feed gas stream to the kindling temperature of the catalyst by indirectly contacting it with products of combustion of fossil fuel, as, for example, described by W. G. Toka and J. R. Burley in Chemical Engineering Practice, Vol. 67, No. 5, p. 57 et seq. (May 1971).
Of course, in a contact sulfuric acid plant operating on a hot sulfur dioxide-containing feed gas stream obtained by combustion of brimstone, sufficient heat is available to preheat the gases prior to introduction into the secondary conversion stage by indirect heat exchange with the hot gas from the brimstone combustion, or by indirect heat exchange contact with converter gases from the first conversion stage, or alternatively, by introducing hot sulfur dioxide-containing gases from the brimstone burner directly into the secondary conversion stage, as described by P. Rinckhoff, in U.S. Pat. No. 3,350,169. The Rinckhoff procedure of introducing hot products of combustion of brimstone directly into the secondary conversion stage seems to be equivalent to Furkert et al.'s procedure of introducing sulfur into that gas stream. In each case, the desired effect is provision of heat by combustion of sulfur. However, the expedient of Rinckhoff et al. is not available in a contact sulfuric acid plant employing a cold sulfur dioxide-containing feed gas stream obtained from a wet gas purification plant.
It is an object of the present invention to provide a double conversion-double absorption contact sulfuric acid process, using cold sulfur dioxide-containing feed gas obtained from a wet gas purification plant, having reduced energy requirements.
It is a further object to provide such process wherein size of equipment required for the primary conversion and intermediate absorption steps is reduced, wherein power requirements to transport the gas stream through the primary conversion and intermediate absorption steps are reduced, and wherein external heat requirements for preheating the feed gas stream to the secondary conversion step are minimized.